1. Field of the Invention
The invention pertains generally to substantially single-crystal material having a composition which includes KTiOPO.sub.4 and to electro-optic devices based thereon.
2. Description of Related Art
Substantially single-crystal KTiOPO.sub.4 (which compound is hereinafter referred to as KTP) is a relatively new material which exhibits many attractive optical properties, including relatively large electro-optic coefficients. As a consequence, this material has been proposed for use in a number of electro-optic devices, in which relatively large electric fields are necessarily employed.
At present, there are two methods available for growing substantially single-crystal KTP. In the first of these methods, known as the hydrothermal technique, a nutrient and a seed crystal are sealed in a gold tube, and the tube is inserted into a high pressure-high temperature autoclave. Crystals are then grown at a relatively high constant pressure of, for example, 25,000 psi and at a constant temperature with a fixed temperature gradient, e.g., 600.degree. Centigrade (C.) at the nutrient end and 550.degree. C. at the seed end. (Regarding the hydrothermal technique see, e.g., G. Gashurov et al, "Growth of KTP", in Tunable Solid State Lasers for Remote Sensing, R. L. Byer, E. K. Gustafson and R. Frebino, eds. (Springer-Verlag, New York, 1985), p. 119). While the resulting KTP crystals are of high quality, it has thus far only been possible to grow relatively small crystals. In addition, the need to employ high pressures has resulted in these crystals being relatively expensive.
The second method available for growing substantially single-crystal KTP is a high-temperature solution growth technique. In this second method, a melt is formed, typically at a temperature of 1050.degree. C., in which reactants capable of reacting to form KTP are dissolved in a solvent having the nominal composition K.sub.6 P.sub.4 O.sub.13. The KTP is crystallized out of solution by initially lowering the melt temperature to, for example, 960.degree. C., inserting a seed crystal of KTP and then cooling the melt at a rate of, for example, 5.degree. C. per day while alternately rotating the seed in the clockwise and counterclockwise directions. (Regarding the high-temperature solution growth technique see P. F. Bordui et al, "Growth of Large Single Crystals of KTiOPO.sub.4 (KTP) From High-Temperature Solution Using Heat Pipe Based Furnace System", Journal of Crystal Growth, Vol. 84, 1987, pp. 403-408). Significantly, this procedure yields relatively large crystals. In addition, because this procedure is carried out at atmospheric pressure, the resulting crystals are relatively inexpensive.
The morphologies of the KTP crystals produced by the hydrothermal technique and the high-temperature solution growth technique are similar. In addition, the optical properties, including the electro-optical properties, are also similar. However, the crystals grown by the two techniques differ significantly in one important electrical property. That is, for the sake of convenience, the 100! crystallographic direction is here defined as the x-axis, the 010! crystallographic direction is here defined as the y-axis and the 001! crystallographic direction is here defined as the z-axis. These definitions correspond to the lattice parameters of the (orthorhombic) crystal structure having space group Pn2a.sub.1, the number of molecules per unit cell being 8, the lattice parameters being a=12.818 Angstroms, b=6.404 Angstroms and c=10.59 Angstroms, and the x-, y- and z- axes being associated with, respectively, the a, b and c lattice parameters. With this definition in mind, it is to be noted that KTP crystals grown via both the hydrothermal and high-temperature solution growth techniques exhibit a conductivity in the z-direction, much higher than that in the x- and y- directions, believed due to the transport of K.sup.+ ions in the z-direction. This transport is, in turn, believed due to the presence of vacancies at the K.sup.+ sites in the crystal lattice. Significantly, the ionic conductivity of a KTP crystal grown via the low-temperature hydrothermal technique is usually relatively low, often as low as about 1.times.10.sup.-9 Siemen's units per centimeter (S/cm). On the other hand, the ionic conductivity of a KTP crystal grown via the high-temperature solution growth technique is usually relatively high, typically being about 2.times.10.sup.-6 S/cm. This difference in z-direction conductivities is important because the electro-optic coefficients associated with the z-direction are relatively large, and therefore, for electro-optic applications, it is often desirable to apply the electric field in the z-direction. But the relatively high z-direction ionic conductivity exhibited by KTP crystals grown via the high-temperature solution growth technique implies that such crystals cannot sustain the relatively high electric fields needed for many electro-optic applications.
Although not specifically directed to lowering ionic conductivities, experiments have been conducted by others in which Ba.sup.+2 dopant ions have been substituted for K.sup.+ ions in KTP crystals. However, rather than lowering the ionic conductivities of the crystals, this substitution increased the ionic conductivities.
Thus, those engaged in developing electro-optic devices have sought, thus far without success, KTP crystals which are relatively large and inexpensive and exhibit relatively low conductivities, particularly in the z-direction.